Target area

Figure 7.6 now shows the stream population for each enthalpy interval together with the hot and cold stream temperatures. Now set up a table to compute Eq. (7.6). This is shown in Table 7.2. Thus the network area target for this problem for = 10°C is 7410 m. ... 

The Fp correction factor for each enthalpy interval depends both on the assumed value of Xp and the temperatures of the interval on the composite curves. It is possible to modify the simple area target formula to obtain the resulting increased overall area A etwork for a network of 1-2 exchangers ... 

Now recalculate the network area target substituting for in Fig. 7.5. Table 7.2 is revised to the values shown in Table 7.3 ... 

The area target is usually slightly less than the area observed in design. 

Townsend, D. W., and Linnhoff, B., Surface Area Targets for Heat Exchanger Networks, IChemE Annual Research Meeting, Bath, U.K., 1984. 

Saboo, A. K., Morari, M., and Colberg, R. D., RESHEX An Interactive Software Package for the Synthesis and Analysis of Resilient Heat Exchanger Networks II. Discussion of Area Targeting and Network Synthesis Algorithms, Computers Chem. Eng., 10 591, 1986. 

Placing the next match above the pinch as shown in Fig. 16.21c also allows the CP inequality to be obeyed. The area for both matches in Fig. 16.21c is 7856 m , and the target for the remaining problem is 1020 m . Accepting both matches causes the overall area target to be exceeded by 17 m (0.2 percent). This seems to be reasonable, and both matches are accepted. No further process-to-process matches are possible, and it remains to place hot utility. 

To establish the shells target, the composite curves are first divided into vertical enthalpy intervals as done for the area target algorithm. It was shown in App. B that it is always possible to design a network for an enthalpy interval with (5, -1) matches, with each match having the same temperature profile as the enthalpy interval. 

The area target, Eq. (7.6), sums the area contributions from each enthalpy interval. This equation can be rearranged to an equivalent expression which sums the area contribution of each stream ... 

Once the factor has been evaluated for each stream, a weighted network area target Anetwork can be calculated ... 

Offset Distance (Nuclear). The distance the desired ground zero or actual ground zero is offset from the center of an area target or from a point target... 

The detailed allocation of fluids to tube-side or shell-side can only be made later in the heat exchanger network design. Also, the area targeting formula does not recognize fluids to be allocated to the tube-side or shell-side. Area targeting only recognizes the individual film heat transfer coefficients. All that can be done in network area targeting... 

Calculate the heat exchange area target for the network. 

Thus, the network area target for this problem for A Tmin = 10°C is 7410 m2. 

Table 17.2 Network area target for the process from Figure 17.2.

A simple algorithm can be developed (see Appendix G) to target the minimum total number of shells (as a real, i.e. noninteger, number) for a stream-set based on the temperature distribution of the composite curves. The algorithm starts by dividing the composite curves into enthalpy intervals in the same way as the area target algorithm. 

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